Peptide Receptor Radionuclide Therapy with 67Cu-CuSarTATE Is Highly Efficacious Against a Somatostatin-Positive Neuroendocrine Tumor Model

A first-in-class dual-chelator theranostic agent designed for use with imaging-therapy radiometal pairs of different elements

A covalent adduct of DFOB and DOTA separated by a l-lysine residue (DFOB-l-Lys-N 6-DOTA) exhibited remarkable regioselective metal binding, with {1H}-13C NMR spectral shifts supporting Zr(iv) coordinating to the DFOB unit, and Lu(iii) coordinating to the DOTA unit. This first-in-class, dual-chelator theranostic design could enable the use of imaging-therapy radiometal pairs of different elements, such as 89Zr for positron emission tomography (PET) imaging and 177Lu for low-energy β--particle radiation therapy. DFOB-l-Lys-N 6-DOTA was elaborated with an amine-terminated polyethylene glycol extender unit (PEG4) to give DFOB-N 2-(PEG4)-l-Lys-N 6-DOTA (compound D2) to enable installation of a phenyl-isothiocyanate group (Ph-NCS) for subsequent monoclonal antibody (mAb) conjugation (mAb = HuJ591). D2-mAb was radiolabeled with 89Zr or 177Lu to produce [89Zr]Zr-D2-mAb or [177Lu]Lu-D2-mAb, respectively, and in vivo PET/CT imaging and in vivo/ex vivo biodistribution properties measured with the matched controls [89Zr]Zr-DFOB-mAb or [177Lu]Lu-DOTA-mAb in a murine LNCaP prostate tumour xenograft model. The 89Zr-immuno-PET imaging function of [89Zr]Zr-D2-mAb and [89Zr]Zr-DFOB-mAb showed no significant difference in tumour accumulation at 48 or 120 h post injection. [89Zr]Zr-D2-mAb and [177Lu]Lu-D2-mAb showed similar ex vivo biodistribution properties at 120 h post-injection. Tumour uptake of [177Lu]Lu-D2-mAb shown by SPECT/CT imaging at 48 h and 120 h post-injection supported the therapeutic function of D2, which was corroborated by similar therapeutic efficacy between [177Lu]Lu-D2-mAb and [177Lu]Lu-DOTA-mAb, both showing a sustained reduction in tumour volume (>80% over 65 d) compared to vehicle. The work identifies D2 as a trifunctional chelator that could expand capabilities in mixed-element radiometal theranostics to improve dosimetry and the clinical outcomes of molecularly targeted radiation.

Copper and Copper Complexes in Tumor Therapy

Copper (Cu), a crucial trace element in physiological processes, has garnered significant interest for its involvement in cancer progression and potential therapeutic applications. The regulation of cellular copper levels is essential for maintaining copper homeostasis, as imbalances can lead to toxicity and cell death. The development of drugs that target copper homeostasis has emerged as a promising strategy for anticancer treatment, with a particular focus on copper chelators, copper ionophores, and novel copper complexes. Recent research has also investigated the potential of copper complexes in cancer therapy.

Effectiveness of [67Cu]Cu-trastuzumab as a theranostic against HER2-positive breast cancer

PURPOSE

To evaluate the imaging and therapeutic properties (theranostic) of 67Cu-labeled anti-human epidermal growth factor receptor II (HER2) monoclonal antibody trastuzumab against HER2-positive breast cancer (BC).

METHODS

We conjugated trastuzumab with p-SCN-Bn-NOTA, 3p-C-NETA-NCS, or p-SCN-Bn-DOTA, and radiolabeled with [67Cu]CuCl2. Immunoconjugate internalization was evaluated in BT-474, JIMT-1 and MCF-7 BC cells. In vitro stability was studied in human serum (HS) and Phosphate Buffered Saline (PBS). Flow cytometry, radioligand binding and immunoreactive fraction assays were carried out. ImmunoSPECT imaging of [67Cu]Cu-NOTA-trastuzumab was done in mice bearing BT-474, JIMT-1 and MCF-7 xenografts. Pharmacokinetic was studied in healthy Balb/c mice while dosimetry was done in both healthy Balb/c and in athymic nude mice bearing JIMT-1 xenograft. The therapeutic effectiveness of [67Cu]Cu-NOTA-trastuzumab was evaluated in mice bearing BT-474 and JIMT-1 xenografts after a single intravenous (i.v.) injection of ~ 16.8 MBq.

RESULTS

Pure immunoconjugates and radioimmunoconjugates (> 95%) were obtained. Internalization was HER2 density-dependent with highest internalization observed with NOTA-trastuzumab. After 5 days, in vitro stabilities were 97 ± 1.7%, 31 ± 6.2%, and 28 ± 4% in HS, and 79 ± 3.5%, 94 ± 1.2%, and 86 ± 2.3% in PBS for [67Cu]Cu-NOTA-trastuzumab, [67Cu]Cu-3p-C-NETA-trastuzumab and [67Cu]Cu-DOTA-trastuzumab, respectively. [67Cu]Cu-NOTA-trastuzumab was chosen for further evaluation. BT-474 flow cytometry showed low KD, 8.2 ± 0.2 nM for trastuzumab vs 26.5 ± 1.6 nM for NOTA-trastuzumab. There were 2.9 NOTA molecules per trastuzumab molecule. Radioligand binding assay showed a low KD of 2.1 ± 0.4 nM and immunoreactive fraction of 69.3 ± 0.9. Highest uptake of [67Cu]Cu-NOTA-trastuzumab was observed in JIMT-1 (33.9 ± 5.5% IA/g) and BT-474 (33.1 ± 10.6% IA/g) xenograft at 120 h post injection (p.i.). Effectiveness of the radioimmunoconjugate was also expressed as percent tumor growth inhibition (%TGI). [67Cu]Cu-NOTA-trastuzumab was more effective than trastuzumab against BT-474 xenografts (78% vs 54% TGI after 28 days), and JIMT-1 xenografts (90% vs 23% TGI after 19 days). Mean survival of [67Cu]Cu-NOTA-trastuzumab, trastuzumab and saline treated groups were > 90, 77 and 72 days for BT-474 xenografts, while that of JIMT-1 were 78, 24, and 20 days, respectively.

CONCLUSION

[67Cu]Cu-NOTA-trastuzumab is a promising theranostic agent against HER2-positive BC.

Efficacy of [67Cu]Cu-EB-TATE Theranostic Against Somatostatin Receptor Subtype-2-Positive Neuroendocrine Tumors

β--emitting 177Lu-octreotate is an approved somatostatin receptor subtype 2 (SSTR2)-directed peptide receptor radionuclide therapy for the treatment of gastroenteropancreatic neuroendocrine tumors (NETs). However,177Lu-octreotate has fast pharmacokinetics, requiring up to 4 treatment doses. Moreover, 177Lu is less than ideal for theranostics because of the low branching ratio of its γ-emissions, which limits its SPECT imaging capability. Compared with 177Lu, 67Cu has better decay properties for use as a theranostic. Here, we report the preclinical evaluation of a long-lived somatostatin analog, [67Cu]Cu-DOTA-Evans blue-TATE (EB-TATE), against SSTR2-positive NETs. Methods: The in vitro cytotoxicity of [67Cu]Cu-EB-TATE was investigated on 2-dimensional cells and 3-dimensional spheroids. In vivo pharmacokinetics and dosimetry were studied in healthy BALB/c mice, whereas ex vivo biodistribution, micro-SPECT/CT imaging, and therapy studies were done on athymic nude mice bearing QGP1.SSTR2 and BON1.SSTR2 xenografts. Therapeutic efficacy was compared with [177Lu]Lu-EB-TATE. Results: Projected human effective doses of [67Cu]Cu-EB-TATE for male (0.066 mSv/MBq) and female (0.085 mSv/MBq) patients are tolerable. In vivo micro-SPECT/CT imaging of SSTR2-positive xenografts with [67Cu]Cu-EB-TATE showed tumor-specific uptake and prolonged accumulation. Biodistribution showed tumor accumulation, with concurrent clearance from major organs over a period of 72 h. [67Cu]Cu-EB-TATE was more effective (60%) at eliminating tumors that were smaller than 50 mm3 within the first 15 d of therapy than was [177Lu]Lu-EB-TATE (20%) after treatment with 2 doses of 15 MBq administered 10 d apart. Mean survival of [67Cu]Cu-EB-TATE-treated groups was 90 d and more than 90 d, whereas that of [177Lu]Lu-EB-TATE was more than 90 d and 89 d against vehicle control groups (26 d and 53 d), for QGP1.SSTR2 and BON1.SSTR2 xenografts, respectively. Conclusion: [67Cu]Cu-EB-TATE exhibited high SSTR2-positive NET uptake and retention, with favorable dosimetry and SPECT/CT imaging capabilities. The antitumor efficacy of [67Cu]Cu-EB-TATE is comparable to that of [177Lu]Lu-EB-TATE, with [67Cu]Cu-EB-TATE being slightly more effective than [177Lu]Lu-EB-TATE for complete remission of small tumors. [67Cu]Cu-EB-TATE therefore warrants clinical development.

Radiolabeling Diaminosarcophagine with Cyclotron-Produced Cobalt-55 and [55Co]Co-NT-Sarcage as a Proof of Concept in a Murine Xenograft Model

Cobalt-sarcophagine complexes exhibit high kinetic inertness under various stringent conditions, but there is limited literature on radiolabeling and in vivo positron emission tomography (PET) imaging using no carrier added 55Co. To fill this gap, this study first investigates the radiolabeling of DiAmSar (DSar) with 55Co, followed by stability evaluation in human serum and EDTA, pharmacokinetics in mice, and a direct comparison with [55Co]CoCl2 to assess differences in pharmacokinetics. Furthermore, the radiolabeling process was successfully used to generate the NTSR1-targeted PET agent [55Co]Co-NT-Sarcage (a DSar-functionalized SR142948 derivative) and administered to HT29 tumor xenografted mice. The [55Co]Co-DSar complex can be formed at 37 °C with purity and stability suitable for preclinical in vivo radiopharmaceutical applications, and [55Co]Co-NT-Sarcage demonstrated prominent tumor uptake with a low background signal. In a direct comparison with [64Cu]Cu-NT-Sarcage, [55Co]Co-NT-Sarcage achieved a higher tumor-to-liver ratio but with overall similar biodistribution profile. These results demonstrate that Sar would be a promising chelator for constructing Co-based radiopharmaceuticals including 55Co for PET and 58mCo for therapeutic applications.

Carrier systems of radiopharmaceuticals and the application in cancer therapy

Radiopharmaceuticals play a vital role in cancer therapy. The carrier of radiopharmaceuticals can precisely locate and guide radionuclides to the target, where radionuclides kill surrounding tumor cells. Effective application of radiopharmaceuticals depends on the selection of an appropriate carrier. Herein, different types of carriers of radiopharmaceuticals and the characteristics are briefly described. Subsequently, we review radiolabeled monoclonal antibodies (mAbs) and their derivatives, and novel strategies of radiolabeled mAbs and their derivatives in the treatment of lymphoma and colorectal cancer. Furthermore, this review outlines radiolabeled peptides, and novel strategies of radiolabeled peptides in the treatment of neuroendocrine neoplasms, prostate cancer, and gliomas. The emphasis is given to heterodimers, bicyclic peptides, and peptide-modified nanoparticles. Last, the latest developments and applications of radiolabeled nucleic acids and small molecules in cancer therapy are discussed. Thus, this review will contribute to a better understanding of the carrier of radiopharmaceuticals and the application in cancer therapy.

HNODThia: A Promising Chelator for the Development of 64Cu Radiopharmaceuticals

Herein, we present the synthesis and coordination chemistry of copper(II) and zinc(II) complexes of two novel heterocyclic triazacyclononane (tacn)-based chelators (HNODThia and NODThia-AcNHEt). The chelator HNODThia was further derivatized to obtain a novel PSMA-based bioconjugate (NODThia-PSMA) and a bifunctional photoactivatable azamacrocyclic analogue, NODThia-PEG3-ArN3, for the development of copper-64 radiopharmaceuticals. 64Cu radiolabeling experiments were performed on the different metal-binding chelates, whereby quantitative radiochemical conversion (RCC) was obtained in less than 10 min at room temperature. The in vitro stability of NODThia-PSMA in human plasma was assessed by ligand-challenge and copper-exchange experiments. Next, we investigated the viability of the photoactivatable analog (NODThia-PEG3-ArN3) for the light-induced photoradiosynthesis of radiolabeled proteins. One-pot photoconjugation reactions to human serum albumin (HSA) as a model protein and the clinically relevant monoclonal antibody formulation MetMAb were performed. [64Cu]Cu-7-azepin-HSA and [64Cu]Cu-7-azepin-onartuzumab were prepared in less than 15 min by irradiation at 395 nm, with radiochemical purities (RCP) of >95% and radiochemical yields (RCYs) of 42.7 ± 5.3 and 49.6%, respectively. Together, the results obtained here open the way for the development of highly stable 64Cu-radiopharmaceuticals by using aza-heterocyclic tacn-based chelators, and the method can easily be extended to the development of 67Cu radiopharmaceuticals for future applications in molecularly targeted radio(immuno)therapy.

Theranostic Nuclear Medicine with Gallium-68, Lutetium-177, Copper-64/67, Actinium-225, and Lead-212/203 Radionuclides

Molecular changes in malignant tissue can lead to an increase in the expression levels of various proteins or receptors that can be used to target the disease. In oncology, diagnostic imaging and radiotherapy of tumors is possible by attaching an appropriate radionuclide to molecules that selectively bind to these target proteins. The term "theranostics" describes the use of a diagnostic tool to predict the efficacy of a therapeutic option. Molecules radiolabeled with γ-emitting or β+-emitting radionuclides can be used for diagnostic imaging using single photon emission computed tomography or positron emission tomography. Radionuclide therapy of disease sites is possible with either α-, β-, or Auger-emitting radionuclides that induce irreversible damage to DNA. This Focus Review centers on the chemistry of theranostic approaches using metal radionuclides for imaging and therapy. The use of tracers that contain β+-emitting gallium-68 and β-emitting lutetium-177 will be discussed in the context of agents in clinical use for the diagnostic imaging and therapy of neuroendocrine tumors and prostate cancer. A particular emphasis is then placed on the chemistry involved in the development of theranostic approaches that use copper-64 for imaging and copper-67 for therapy with functionalized sarcophagine cage amine ligands. Targeted therapy with radionuclides that emit α particles has potential to be of particular use in late-stage disease where there are limited options, and the role of actinium-225 and lead-212 in this area is also discussed. Finally, we highlight the challenges that impede further adoption of radiotheranostic concepts while highlighting exciting opportunities and prospects.

Recent Pre-Clinical Advancements in Nuclear Medicine: Pioneering the Path to a Limitless Future

The theranostic approach in oncology holds significant importance in personalized medicine and stands as an exciting field of molecular medicine. Significant achievements have been made in this field in recent decades, particularly in treating neuroendocrine tumors using 177-Lu-radiolabeled somatostatin analogs and, more recently, in addressing prostate cancer through prostate-specific-membrane-antigen targeted radionuclide therapy. The promising clinical results obtained in these indications paved the way for the further development of this approach. With the continuous discovery of new molecular players in tumorigenesis, the development of novel radiopharmaceuticals, and the potential combination of theranostics agents with immunotherapy, nuclear medicine is poised for significant advancements. The strategy of theranostics in oncology can be categorized into (1) repurposing nuclear medicine agents for other indications, (2) improving existing radiopharmaceuticals, and (3) developing new theranostics agents for tumor-specific antigens. In this review, we provide an overview of theranostic development and shed light on its potential integration into combined treatment strategies.

Optimization of Deuteron Irradiation of 176Yb for Producing 177Lu of High Specific Activity Exceeding 3000 GBq/mg

The irradiation of 176Yb with deuterons offers a promising pathway for the production of the theranostic radionuclide 177Lu. To optimize this process, calculations integrating deuteron transport, isotope production, and decay have been performed. In pure 176Yb, the undesired production of 174g+mLu occurs at higher deuteron energies, corresponding to a distribution slightly shallower than that of 177Lu. Hence, 174g+mLu can be effectively filtered out by employing either a low-energy deuteron beam or stacked foils. The utilization of stacked foils enables the production of 177Lu using a high-energy linear accelerator. Another unwanted isotope, 176mLu, is produced roughly at the same depth as 177Lu, but its concentration can be significantly reduced by selecting an appropriate post-irradiation processing time, owing to its relatively short half-life. The modeling approach extended to the mapping of yields as a function of irradiation time and post-irradiation processing time. An optimized processing time window was identified. The study demonstrates that a high-energy deuteron beam can be employed to produce 177Lu with high specific activity exceeding 3000 GBq/mg. The effect of different purity levels (ranging from 98% to 100%) was also discussed. The impurity levels have a slight impact. The modeling demonstrates the feasibility of obtaining 177Lu with a specific activity > 3000 GBq/mg and radionuclidic purity > 99.5% when using a commercially available 176Yb target of 99.6% purity.

Molecular imaging Theranostics of Neuroendocrine Tumors

Neuroendocrine neoplasms (NEN) are rare and heterogeneous tumors, originating mostly from the gastro-entero-pancreatic (GEP) tract followed by the lungs. Multidisciplinary discussion is mandatory for optimal diagnostic and therapeutic management. Well-differentiated NEN (NET) present a high expression of somatostatin receptors (SSTR) and can be studied with [68Ga]-DOTA-peptides ([68Ga]Ga-DOTANOC, [68Ga]Ga-DOTATOC, [68Ga]Ga-DOTATATE) PET/CT to assess disease extension and the eligibility for peptide receptor radionuclide therapy (PRRT). SSTR-analogues labelled with 90Y or 177Lu have been used since mid-90s for NET therapy. PRRT is now considered an effective and safe treatment option for SSTR-expressing NET: following the approval of 177Lu-DOTATATE by FDA and EMA, PRRT is now part of the therapeutic algorithms of the main scientific societies. New strategies to improve PRRT efficacy and to reduce its toxicity are under evaluation (eg, personalization of treatment schemes, the selection of the most suitable patients, improvement of response assessment criteria, optimization of treatment sequencing, feasibility of PRRT-retreatment, combination of PRRT with other treatments options). Recently, several emerging radiopharmaceuticals showed encouraging results for both imaging and therapy (eg, SSTR-analogues labelled with 18F, SSTR-antagonists for both diagnosis and therapy, alpha-labelling for therapy, radiopharmaceuticals binding to new cellular targets). Aim of this review is to focus on current knowledge and to outline emerging perspectives for NEN's diagnosis and therapy.

Recent Advances in 64Cu/67Cu-Based Radiopharmaceuticals

Copper-64 (T_1/2 = 12.7 h) is a positron and beta-emitting isotope, with decay characteristics suitable for both positron emission tomography (PET) imaging and radiotherapy of cancer. Copper-67 (T_1/2 = 61.8 h) is a beta and gamma emitter, appropriate for radiotherapy β-energy and with a half-life suitable for single-photon emission computed tomography (SPECT) imaging. The chemical identities of ⁶⁴Cu and ⁶⁷Cu isotopes allow for convenient use of the same chelating molecules for sequential PET imaging and radiotherapy. A recent breakthrough in ⁶⁷Cu production opened previously unavailable opportunities for a reliable source of ⁶⁷Cu with high specific activity and purity. These new opportunities have reignited interest in the use of copper-containing radiopharmaceuticals for the therapy, diagnosis, and theranostics of various diseases. Herein, we summarize recent (2018-2023) advances in the use of copper-based radiopharmaceuticals for PET, SPECT imaging, radiotherapy, and radioimmunotherapy.

Introduction of a Polyethylene Glycol Linker Improves Uptake of 67Cu-NOTA-Conjugated Lactam-Cyclized Alpha-Melanocyte-Stimulating Hormone Peptide in Melanoma

The aim of this study was to evaluate the effect of linker on tumor targeting and biodistribution of ⁶⁷Cu-NOTA-PEG₂Nle-CycMSH_hex {⁶⁷Cu-1,4,7-triazacyclononane-1,4,7-triyl-triacetic acid-polyethylene glycol-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-CONH₂} and ⁶⁷Cu-NOTA-GGNle-CycMSH_hex {⁶⁷Cu-NOTA-GlyGlyNle-CycMSH_hex} on melanoma-bearing mice. NOTA-PEG₂Nle-CycMSH_hex and NOTA-GGNle-CycMSH_hex were synthesized and purified by HPLC. The biodistribution of ⁶⁷Cu-NOTA-PEG₂Nle-CycMSH_hex and ⁶⁷Cu-NOTA-GGNle-CycMSH_hex was determined in B16/F10 melanoma-bearing C57 mice. The melanoma imaging property of ⁶⁷Cu-NOTA-PEG₂Nle-CycMSH_hex was further examined in B16/F10 melanoma-bearing C57 mice. ⁶⁷Cu-NOTA-PEG₂Nle-CycMSH_hex exhibited higher tumor uptake than ⁶⁷Cu-NOTA-GGNle-CycMSH_hex at 2, 4, and 24 h post-injection. The tumor uptake of ⁶⁷Cu-NOTA-PEG₂Nle-CycMSH_hex was 27.97 ± 1.98, 24.10 ± 1.83, and 9.13 ± 1.66% ID/g at 2, 4, and 24 h post-injection, respectively. Normal organ uptake of ⁶⁷Cu-NOTA-PEG₂Nle-CycMSH_hex was lower than 2.6% ID/g at 4 h post-injection, except for kidney uptake. The renal uptake of ⁶⁷Cu-NOTA-PEG₂Nle-CycMSH_hex was 6.43 ± 1.31, 2.60 ± 0.79, and 0.90 ± 0.18% ID/g at 2, 4, and 24 h post-injection, respectively. ⁶⁷Cu-NOTA-PEG₂Nle-CycMSH_hex showed high tumor to normal organ uptake ratios after 2 h post-injection. The B16/F10 melanoma lesions could be clearly visualized by single photon emission computed tomography (SPECT) using ⁶⁷Cu-NOTA-PEG₂Nle-CycMSH_hex as an imaging probe at 4 h post-injection. The favorable tumor targeting and biodistribution properties of ⁶⁷Cu-NOTA-PEG₂Nle-CycMSH_hex underscored its potential as an MC1R-targeted therapeutic peptide for melanoma treatment.

N,N-Alkylation Clarifies the Role of N- and O-Protonated Intermediates in Cyclen-Based 64Cu Radiopharmaceuticals

Radioisotopes of Cu, such as ⁶⁴Cu and ⁶⁷Cu, are alluring targets for imaging (e.g., positron emission tomography, PET) and radiotherapeutic applications. Cyclen-based macrocyclic polyaminocarboxylates are one of the most frequently examined bifunctional chelators in vitro and in vivo, including the FDA-approved ⁶⁴Cu radiopharmaceutical, Cu(DOTATATE) (Detectnet); however, connections between the structure of plausible reactive intermediates and their stability under physiologically relevant conditions remain to be established. In this study, we share the synthesis of a cyclen-based, N,N-alkylated spirocyclic chelate, H₂DO3A^C4H8, which serves as a model for N-protonation. Our combined experimental (in vitro and in vivo) and computational studies unravel complex pH-dependent speciation and enable side-by-side comparison of N- and O-protonated species of relevant ⁶⁴Cu radiopharmaceuticals. Our studies suggest that N-protonated species are not inherently unstable species under physiological conditions and demonstrate the potential of N,N-alkylation as a tool for the rational design of future radiopharmaceuticals.

Head-to-Head Comparison between Peptide-Based Radiopharmaceutical for PET and SPECT in the Evaluation of Neuroendocrine Tumors: A Systematic Review

We compared head-to-head the most used radiolabeled peptides for single photon computed emission tomography (SPECT) and positron emission tomography (PET) imaging of neuroendocrine tumors (NETs). A comprehensive literature search was performed in PubMed, Web of Science, and Scopus databases. The following words, coupled two by two, were used: 68Ga-DOTATOC; 68Ga-DOTATATE; 68Ga-DOTANOC; 99mTc-EDDA/HYNIC-TOC; 64Cu-DOTATATE; and 111In-DTPA-octreotide. Moreover, a second-step search strategy was adopted by using the following combined terms: "Somatostatin receptor imaging,"; "Somatostatin receptor imaging" and "Functional,"; "Somatostatin receptor imaging" and "SPECT,"; and "Somatostatin receptor imaging" and "PET". Eligible criteria were: (1) original articles focusing on the clinical application of the radiopharmaceutical agents in NETs; (2) original articles in the English language; (3) comparative studies (head-to-head comparative or matched-paired studies). Editorials, letters to the editor, reviews, pictorial essays, clinical cases, or opinions were excluded. A total of 1077 articles were found in the three electronic databases. The full texts of 104 articles were assessed for eligibility. Nineteen articles were finally included. Most articles focused on the comparison between 111In-DTPA-Octreotide and 68Ga-DOTATOC/TATE. Few papers compared 64Cu-DOTATATE and 68Ga-DOTATOC/TATE, or SPECT tracers. The rates of true positivity were 63.7%, 58.5%, 78.4% and 82.4%, respectively, for 111In-DTPA-Octreotide, 99mTc-EDDA/HYNIC-TOC, 68Ga-DOTATATE/TOC and 64Cu-DOTATATE. In conclusion, as highly expected, PET tracers are more suitable for the in vivo identification of NETs. Indeed, in comparative studies, they demonstrated a higher true positive rate than SPECT agents.

Comparison of the Anti-Tumour Activity of the Somatostatin Receptor (SST) Antagonist [177Lu]Lu-Satoreotide Tetraxetan and the Agonist [177Lu]Lu-DOTA-TATE in Mice Bearing AR42J SST2-Positive Tumours

Limited experiments have compared the treatment effects of repetitive cycles of radiolabelled somatostatin (SST) analogues. In vitro and in vivo experiments were conducted in an AR42J cancer cell model, comparing the antagonist [177Lu]Lu-satoreotide tetraxetan with the agonist [177Lu]Lu-DOTA-TATE in terms of their binding properties, biodistribution, anti-tumour activity and toxicity. Histopathological and immunohistochemical examinations were performed at different timepoints. In the in vitro assays, [177Lu]Lu-satoreotide tetraxetan recognised twice as many SST2 binding sites as [177Lu]Lu-DOTA-TATE. In mice treated once a week for four consecutive weeks, [177Lu]Lu-satoreotide tetraxetan (15 MBq) revealed a significantly greater median time taken to reach a tumour volume of 850 mm3 (68 days) compared to [177Lu]Lu-DOTA-TATE at 15 MBq (43 days) or 30 MBq (48 days). This was associated with a higher tumour uptake, enhanced DNA damage and no or mild effects on body weight, haematological toxicity, or renal toxicity with [177Lu]Lu-satoreotide tetraxetan (15 MBq). At the end of the study, complete tumour senescence was noted in 20% of animals treated with [177Lu]Lu-satoreotide tetraxetan, in 13% of those treated with [177Lu]Lu-DOTA-TATE at 30 MBq, and in none of those treated with [177Lu]Lu-DOTA-TATE at 15 MBq. In conclusion, repeated administrations of [177Lu]Lu-satoreotide tetraxetan were able to potentiate peptide receptor radionuclide therapy with a higher tumour uptake, longer median survival, and enhanced DNA damage, with a favourable efficacy/safety profile compared to [177Lu]Lu-DOTA-TATE.

Production Review of Accelerator-Based Medical Isotopes

The production of reactor-based medical isotopes is fragile, which has meant supply shortages from time to time. This paper reviews alternative production methods in the form of cyclotrons, linear accelerators and neutron generators. Finally, the status of the production of medical isotopes in China is described.

Structural insights into the activation of somatostatin receptor 2 by cyclic SST analogues

The endogenous cyclic tetradecapeptide SST14 was reported to stimulate all five somatostatin receptors (SSTR1-5) for hormone release, neurotransmission, cell growth arrest and cancer suppression. Two SST14-derived short cyclic SST analogues (lanreotide or octreotide) with improved stability and longer lifetime were developed as drugs to preferentially activate SSTR2 and treat acromegalia and neuroendocrine tumors. Here, cryo-EM structures of the human SSTR2-Gi complex bound with SST14, octreotide or lanreotide were determined at resolutions of 2.85 Å, 2.97 Å, and 2.87 Å, respectively. Structural and functional analysis revealed that interactions between β-turn residues in SST analogues and transmembrane SSTR2 residues in the ligand-binding pocket are crucial for receptor binding and functional stimulation of the two SST14-derived cyclic octapeptides. Additionally, Q102^2.63, N276^6.55, and F294^7.35 could be responsible for the selectivity of lanreotide or octreotide for SSTR2 over SSTR1 or SSTR4. These results provide valuable insights into further rational development of SST analogue drugs targeting SSTR2.

Non-invasive radionuclide imaging of trace metal trafficking in health and disease: "PET metallomics"

Several specific metallic elements must be present in the human body to maintain health and function. Maintaining the correct quantity (from trace to bulk) and location at the cell and tissue level is essential. The study of the biological role of metals has become known as metallomics. While quantities of metals in cells and tissues can be readily measured in biopsy and autopsy samples by destructive analytical techniques, their trafficking and its role in health and disease are poorly understood. Molecular imaging with radionuclides - positron emission tomography (PET) and single photon emission computed tomography (SPECT) - is emerging as a means to non-invasively study the acute trafficking of essential metals between organs, non-invasively and in real time, in health and disease. PET scanners are increasingly widely available in hospitals, and methods for producing radionuclides of some of the key essential metals are developing fast. This review summarises recent developments in radionuclide imaging technology that permit such investigations, describes the radiological and physicochemical properties of key radioisotopes of essential trace metals and useful analogues, and introduces current and potential future applications in preclinical and clinical investigations to study the biology of essential trace metals in health and disease.

The Current Role of Peptide Receptor Radionuclide Therapy in Meningiomas

Meningiomas are the most common primary intracranial tumors. The majority of patients can be cured by surgery, or tumor growth can be stabilized by radiation. However, the management of recurrent and more aggressive tumors remains difficult because no established alternative treatment options exist. Therefore, innovative therapeutic approaches are needed. Studies have shown that meningiomas express somatostatin receptors. It is well known from treating neuroendocrine tumors that peptide radioreceptor therapy that targets somatostatin receptors can be effective. As yet, this therapy has been used for treating meningiomas only within individual curative trials. However, small case series and studies have demonstrated stabilization of the disease. Therefore, we see potential for optimizing this therapeutic option through the development of new substances and specific adaptations to the different meningioma subtypes. The current review provides an overview of this topic.

A feasibility study of the therapeutic application of a mixture of 67/64 Cu radioisotopes produced by cyclotrons with proton irradiation

PURPOSE

64 Cu and 67 Cu radioisotopes have nuclear characteristics suitable for nuclear medicine applications. The production of 64 Cu is already well established. However, the production of 67 Cu in quantities suitable to conduct clinical trials is more challenging as it leads to the coproduction of other Cu isotopes, in particular 64 Cu. The aim of this study is to investigate the possibility of using a CuCl2 solution with a mixture of 67/64 Cu radioisotopes for therapeutic purposes, providing an alternative solution for the cyclotron production problem.

METHODS

Copper radioisotopes activities were calculated by considering proton beam irradiation of the following targets: (i) 70 Zn in the energy range 70-45 MeV; (ii) 68 Zn in the energy range 70-35 MeV; (iii) a combination of 70 Zn (70-55 MeV) and 68 Zn (55-35 MeV). The contribution of each copper radioisotope to the human-absorbed dose was estimated with OLINDA/EXM software using the biokinetic model for CuCl2 published by ICRP 53. The total absorbed dose generated by the 67/64 CuCl2 mixture, obtained through different production routes, was calculated at different times after the end of the bombardment (EOB). A simple spherical model was used to simulate tumors of different sizes containing uniformly distributed 67/64 Cu mixture and to calculate the absorbed dose of self-irradiation. The biological damage produced by 67 Cu and 64 Cu was also evaluated through cellular dosimetry and cell surviving fraction assessment using the MIRDcell code, considering two prostate cancer cell lines with different radiosensitivity.

RESULTS

The absorbed dose to healthy organs and the effective dose (ED) per unit of administered activity of 67 CuCl2 are higher than those of 64 CuCl2 . Absorbed dose values per unit of administered activity of 67/64 CuCl2 mixture increase with time after the EOB because the amount of 67 Cu in the mixture increases. Survival data showed that the biological damage caused per each decay of 67 Cu is greater than that of 64 Cu, assuming that radionuclides remain accumulated in the cell cytoplasm. Sphere model calculations demonstrated that 64 Cu administered activity must be about five times higher than that of 67 Cu to obtain the same absorbed dose for tumor mass between 0.01 and 10 g and about 10 times higher for very small spheres. Consequently, the 64 CuCl2 -absorbed dose to healthy organs will reach higher values than those of 67 CuCl2 . The supplemental activity of the 67/64 CuCl2 mixture, required to get the same tumor-absorbed dose produced by 67 CuCl2 , triggers a dose increment (DI) in healthy organs. The waiting time post-EOB necessary to keep this DI below 10% (t10% ) depends on the irradiation methods employed for the production of the 67/64 CuCl2 mixture.

CONCLUSIONS

A mixture of cyclotron produced 67/64 Cu radioisotopes proved to be an alternative solution for the therapeutic use of CuCl2 with minimal DI to healthy organs compared with pure 67 Cu. Irradiation of a 70 Zn+68 Zn target in the 70-35 MeV proton energy range for 185 h appears to be the best option from among all the production routes investigated, as it gives the maximum amount of activity, the shortest t10% (10 h), and less than 1% of 61 Cu and 60 Cu impurities.

New PET Radiotracers for the Imaging of Neuroendocrine Neoplasms

Neuroendocrine neoplasms (NEN) are a heterogeneous group of tumours derived from cells of neuroendocrine origin and can potentially arise everywhere in the human body. The diagnostic assessment of NEN can be performed using a variety of PET radiopharmaceuticals. Well-differentiated NEN (NET) present a high expression of SSTR (somatostatin receptors) and can therefore be studied with 68Ga-DOTA-peptides ([68Ga]Ga-DOTANOC, [68Ga]Ga-DOTATOC, [68Ga]Ga-DOTATATE). Current guidelines recommend the use of SSTR imaging to assess disease extension at staging/restaging, follow-up, assessment of response to therapy and selection of patients who may benefit from radionuclide therapy (PRRT). [18F]F-FDG is used for the assessment of high-grade tumours (high-grade G2, G3 and NEC) and in every case, there is one or more mismatched lesions between diagnostic CT (positive) and SSTR-PET/CT (negative). [18F]F-DOPA is currently used for the assessment of medullary thyroid carcinoma, neuroblastoma, primary pheochromocytoma and abdominal paraganglioma. In recent years, however, several new tracers were designed exploiting the many potential targets of the neuroendocrine cell and were employed in clinical trials for both imaging and therapy. Currently, the real-life clinical impact of these tracers is still mostly not known; however, the favourable biodistribution (e.g. [68Ga]Ga-FAPI, SSTR antagonists) and the possibility to use new theranostic pairs may provide novel diagnostic as well as therapeutic options (e.g. [68Ga]Ga-PSMA, [64Cu]Cu-SARTATE, [68Ga]Ga-CXCR4) for NEN patients.

67Cu Production Capabilities: A Mini Review

Is the ⁶⁷Cu production worldwide feasible for expanding preclinical and clinical studies? How can we face the ingrowing demands of this emerging and promising theranostic radionuclide for personalized therapies? This review looks at the different production routes, including the accelerator- and reactor-based ones, providing a comprehensive overview of the actual ⁶⁷Cu supply, with brief insight into its use in non-clinical and clinical studies. In addition to the most often explored nuclear reactions, this work focuses on the ⁶⁷Cu separation and purification techniques, as well as the target material recovery procedures that are mandatory for the economic sustainability of the production cycle. The quality aspects, such as radiochemical, chemical, and radionuclidic purity, with particular attention to the coproduction of the counterpart ⁶⁴Cu, are also taken into account, with detailed comparisons among the different production routes. Future possibilities related to new infrastructures are included in this work, as well as new developments on the radiopharmaceuticals aspects.

Treatment of Neuroendocrine Neoplasms with Radiolabeled Peptides-Where Are We Now

Peptide receptor radionuclide therapy (PRRT) has been one of the most successful and exciting examples of theranostics in nuclear medicine in recent decades and is now firmly embedded in many treatment algorithms for unresectable or metastatic neuroendocrine neoplasms (NENs) worldwide. It is widely considered to be an effective treatment for well- or moderately differentiated neoplasms, which express high levels of somatostatin receptors that can be selectively targeted. This review article outlines the scientific basis of PRRT in treatment of NENs and describes its discovery dating back to the early 1990s. Early treatments utilizing Indium-111, a γ-emitter, showed promise in reduction in tumor size and improvement in biochemistry, but were also met with high radiation doses and myelotoxic and nephrotoxic effects. Subsequently, stable conjugation of DOTA-peptides with β-emitting radionuclides, such as Yttrium-90 and Lutetium-177, served as a breakthrough for PRRT and studies highlighted their potential in eliciting progression-free survival and quality of life benefits. This article will also elaborate on the key trials which paved the way for its approval and will discuss therapeutic considerations, such as patient selection and administration technique, to optimize its use.

Hyperfractionated Treatment with 177Lu-Octreotate Increases Tumor Response in Human Small-Intestine Neuroendocrine GOT1 Tumor Model

Simple Summary

Neuroendocrine tumors are slow growing and initially associated with vague symptoms and, therefore, often spread in the patient’s body at diagnosis, leading to a poor prognosis without means of curation through surgery. Although tumor-targeting treatments exist and are used in clinics, they are not fully optimized. The aim of this study was to test different dosages and time intervals of the radioactive pharmaceutical ¹⁷⁷Lu-octreotate. We found that dividing a dosage into several portions and administering it at short time intervals resulted in a stronger tumor reduction and/or prolonged time for regrowth in mice than if given as a single dose. The biggest differences were seen in the lower dosage levels of the study. The findings indicate that there is clear room for improvements in the treatment of neuroendocrine tumors with ¹⁷⁷Lu-octreotate.

Abstract

Radionuclide treatment of patients with neuroendocrine tumors has advanced in the last decades with favorable results using ¹⁷⁷Lu-octreotate. However, the gap between the high cure rate in animal studies vs. patient studies indicates a potential to increase the curation of patients. The aim of this study was to investigate the tumor response for different fractionation schemes with ¹⁷⁷Lu-octreotate. BALB/c mice bearing a human small-intestine neuroendocrine GOT1 tumor were either mock treated with saline or injected intravenously with a total of 30–120 MBq of ¹⁷⁷Lu-octreotate: 1 × 30, 2 × 15, 1 × 60, 2 × 30, 1 × 120, 2 × 60, or 3 × 40 MBq. The tumor volume was measured twice per week until the end of the experiment. The mean tumor volume for mice that received 2 × 15 = 30 and 1 × 30 MBq ¹⁷⁷Lu-octreotate was reduced by 61% and 52%, respectively. The mean tumor volume was reduced by 91% and 44% for mice that received 2 × 30 = 60 and 1 × 60 MBq ¹⁷⁷Lu-octreotate, respectively. After 120 MBq ¹⁷⁷Lu-octreotate, given as 1–3 fractions, the mean tumor volume was reduced by 91–97%. Multiple fractions resulted in delayed regrowth and prolonged overall survival by 20–25% for the 120 MBq groups and by 45% for lower total activities, relative to one fraction. The results indicate that fractionation and hyperfractionation of ¹⁷⁷Lu-octreotate are beneficial for tumor reduction and prolongs the time to regrowth.

Photoactivatable bis(thiosemicarbazone) derivatives for copper-64 radiotracer synthesis

In recent years, copper-64 and copper-67 have been considered as a useful theranostic pair in nuclear medicine, due to their favourable and complementary decay properties. As ⁶⁷Cu and ⁶⁴Cu are chemically identical, development of both existing and new bifunctional chelators for ⁶⁴Cu imaging agents can be readily adapted for the ⁶⁷Cu-radionuclide. In this study, we explored the use of photoactivatable copper chelators based on the asymmetric bis(thiosemicarbazone) scaffold, H₂ATSM/en, for the photoradiolabelling of protein. Photoactivatable ⁶⁴CuATSM-derivatives were prepared by both direct synthesis and transmetallation from the corresponding ^natZn complex. Then, irradiation with UV light in the presence of a protein of interest in a pH buffered aqueous solution afforded the ⁶⁴Cu-labelled protein conjugates in decay-corrected radiochemical yield of 86.9 ± 1.0% via the transmetallation method and 35.3 ± 1.7% from the direct radiolabelling method. This study successfully demonstrates the viability of photochemically induced conjugation methods for the development of copper-based radiotracers for potential applications in diagnostic positron emission tomography (PET) imaging and targeted radionuclide therapy.

In recent years, copper-64 and copper-67 have been considered as a useful theranostic pair in nuclear medicine. Here, we report a photochemically-mediated approach for radiolabelling biologically relevant protein with copper radionuclides.

Enzyme mediated incorporation of zirconium-89 or copper-64 into a fragment antibody for same day imaging of epidermal growth factor receptor

Identification of tumors which over-express Epidermal Growth Factor Receptor (EGFR) is important in selecting patients for anti-EGFR therapies. Enzymatic bioconjugation was used to introduce positron-emitting radionuclides (89Zr, 64Cu) into an anti-EGFR antibody fragment for Positron Emission Tomography (PET) imaging the same day as injection. A monovalent antibody fragment with high affinity for EGFR was engineered to include a sequence that is recognized by the transpeptidase sortase A. Two different metal chelators, one for 89ZrIV and one for 64CuII, were modified with a N-terminal glycine to enable them to act as substrates in sortase A mediated bioconjugation to the antibody fragment. Both fragments provided high-quality PET images of EGFR positive tumors in a mouse model at 3 hours post-injection, a significant advantage when compared to radiolabeled full antibodies that require several days between injection of the tracer and imaging. The use of enzymatic bioconjugation gives reproducible homogeneous products with the metal complexes selectively installed on the C-terminus of the antibody potentially simplifying regulatory approval.

Therapeutic Efficacy of a Bivalent Inhibitor of Prostate-Specific Membrane Antigen Labeled with 67Cu

Radionuclide therapy targeting prostate-specific membrane antigen (PSMA) is promising for prostate cancer. We previously reported a ligand, ⁶⁴Cu-CuSarbisPSMA, featuring 2 lysine-ureido-glutamate groups. Here, we report the therapeutic potential of ⁶⁷Cu-CuSarbisPSMA. Methods: Growth of PSMA-positive xenografts was evaluated after treatment with ⁶⁷Cu-CuSarbisPSMA or ¹⁷⁷Lu-LuPSMA imaging and therapy (I&T). Results: At 13 d after injection, tumor growth was similarly inhibited by the 2 tracers in a dose-dependent manner. Survival was comparable after single (30 MBq) or fractionated (2 × 15 MBq, 2 wk apart) administrations. Conclusion: ⁶⁷Cu-CuSarbisPSMA is efficacious in a PSMA-expressing model of prostate cancer.

Imaging Somatostatin Positive Tumors with Tyr3-Octreotate/Octreotide Conjugated to Desferrioxamine B Squaramide Radiolabeled with either Zirconium-89 or Gallium-68

Radiolabeled derivatives of Tyr³-octreotide and Tyr³-octreotate, synthetic analogues of the peptide hormone somatostatin, can be used for positron emission tomography (PET) imaging of somatostatin receptor expression in neuroendocrine tumors. In this work, a squaramide ester derivative of desferrioxamine B (H₃DFOSq) was used attach either Tyr³-octreotide or Tyr³-octreotate to the metal binding ligand to give H₃DFOSq-TIDE and H₃DFOSq-TATE. These new peptide-H₃DFOSq conjugates form stable complexes with either of the positron-emitting radionuclides gallium-68 (t_1/2 = 68 min) or zirconium-89 (t_1/2 = 3.3 days). The new complexes were evaluated in an AR42J xenograft model that has endogenous expression of SSTR2. All four agents displayed good tumor uptake and produced high-quality PET images. For both radionuclides, the complexes formed with H₃DFOSq-TATE performed better, with higher tumor uptake and retention than the complexes formed with H₃DFOSq-TIDE. The versatile ligands presented here can be radiolabeled with either gallium-68 or zirconium-89 at room temperature. The long radioactive half-life of zirconium-89 makes distribution of pre-synthesized tracers produced to certified standards feasible and could increase the number of clinical centers that can perform diagnostic PET imaging of neuroendocrine tumors.

Strategies Towards Improving Clinical Outcomes of Peptide Receptor Radionuclide Therapy

PURPOSE OF REVIEW

Peptide receptor radionuclide therapy (PRRT) with [177Lu-DOTA0,Tyr3] octreotate is an effective and safe second- or third-line treatment option for patients with low-grade advanced gastroenteropancreatic (GEP) neuroendocrine neoplasms (NEN). In this review, we will focus on possible extensions of the current use of PRRT and on new approaches which could further improve its treatment efficacy and safety.

RECENT FINDINGS

Promising results were published regarding PRRT in other NENs, including lung NENs or high-grade NENs, and applying PRRT as neoadjuvant or salvage therapy. Furthermore, a diversity of strategic approaches, including dosimetry, somatostatin receptor antagonists, somatostatin receptor upregulation, radiosensitization, different radionuclides, albumin binding, alternative renal protection, and liver-directed therapy in combination with PRRT, have the potential to improve the outcome of PRRT. Also, novel biomarkers are presented that could predict response to PRRT. Multiple preclinical and early clinical studies have shown encouraging potential to advance the clinical outcome of PRRT in NEN patients. However, at this moment, most of these strategies have not yet reached the clinical setting of randomized phase III trials.

Detection and therapy of neuroblastoma minimal residual disease using [64/67Cu]Cu-SARTATE in a preclinical model of hepatic metastases

Background

A major challenge to the long-term success of neuroblastoma therapy is widespread metastases that survive initial therapy as minimal residual disease (MRD). The SSTR2 receptor is expressed by most neuroblastoma tumors making it an attractive target for molecularly targeted radionuclide therapy. SARTATE consists of octreotate, which targets the SSTR2 receptor, conjugated to MeCOSar, a bifunctional chelator with high affinity for copper. Cu-SARTATE offers the potential to both detect and treat neuroblastoma MRD by using [⁶⁴Cu]Cu-SARTATE to detect and monitor the disease and [⁶⁷Cu]Cu-SARTATE as the companion therapeutic agent. In the present study, we tested this theranostic pair in a preclinical model of neuroblastoma MRD. An intrahepatic model of metastatic neuroblastoma was established using IMR32 cells in nude mice. The biodistribution of [⁶⁴Cu]Cu-SARTATE was measured using small-animal PET and ex vivo tissue analysis. Survival studies were carried out using the same model: mice (6–8 mice/group) were given single doses of saline, or 9.25 MBq (250 µCi), or 18.5 MBq (500 µCi) of [⁶⁷Cu]Cu-SARTATE at either 2 or 4 weeks after tumor cell inoculation.

Results

PET imaging and ex vivo biodistribution confirmed tumor uptake of [⁶⁴Cu]Cu-SARTATE and rapid clearance from other tissues. The major clearance tissues were the kidneys (15.6 ± 5.8% IA/g at 24 h post-injection, 11.5 ± 2.8% IA/g at 48 h, n = 3/4). Autoradiography and histological analysis confirmed [⁶⁴Cu]Cu-SARTATE uptake in viable, SSTR2-positive tumor regions with mean tumor uptakes of 14.1–25.0% IA/g at 24 h. [⁶⁷Cu]Cu-SARTATE therapy was effective when started 2 weeks after tumor cell inoculation, extending survival by an average of 13 days (30%) compared with the untreated group (mean survival of control group 43.0 ± 8.1 days vs. 55.6 ± 9.1 days for the treated group; p = 0.012). No significant therapeutic effect was observed when [⁶⁷Cu]Cu-SARTATE was started 4 weeks after tumor cell inoculation, when the tumors would have been larger (control group 14.6 ± 8.5 days; 9.25 MBq group 9.5 ± 1.6 days; 18.5 MBq group 15.6 ± 4.1 days; p = 0.064).

Conclusions

Clinical experiences of peptide-receptor radionuclide therapy for metastatic disease have been encouraging. This study demonstrates the potential for a theranostic approach using [^64/67Cu]Cu-SARTATE for the detection and treatment of SSTR2-positive neuroblastoma MRD.

Copper-67 radioimmunotheranostics for simultaneous immunotherapy and immuno-SPECT

Copper-67 (t1/2 = 2.58 days) decays by β- ([Formula: see text]: 562 keV) and γ-rays (93 keV and 185 keV) rendering it with potential for both radionuclide therapy and single-photon emission computed tomography (SPECT) imaging. Prompted by the recent breakthrough of 67Cu production with high specific activity, high radionuclidic purity, and sufficient quantities, the interest in the theranostic potential of 67Cu has been rekindled. This work addresses the practicability of developing 67Cu-labeled antibodies with substantially improved quality for cancer radioimmunotheranostics. Proof of concept is demonstrated with pertuzumab, a US-FDA-approved monoclonal antibody for combination therapies of HER2-positive breast cancer. With an average number of 1.9 chelators coupled to each antibody, we achieved a two-order of magnitude increase in radiolabeling efficiency compared to literature reports. In a preclinical therapeutic study, mice (n = 4-7/group) bearing HER2+ xenografts exhibited a 67Cu-dose dependent tumor-growth inhibition from 67Cu-labeled-Pertuzumab co-administered with trastuzumab. Furthermore, greater tumor size reduction was observed with 67Cu-labeled-pertuzumab formulations of higher specific activity. The potential of SPECT imaging with 67Cu radiopharmaceuticals was tested after 67Cu-labeled-Pertuzumab administration. Impressively, all tumors were clearly visualized by SPECT imaging with 67Cu-labeled-Pertuzumab even at day 5 post injection. This work demonstrates it is practical to use 67Cu radioimmunoconjugates for cancer radioimmunotheranostics.

Harnessing 64Cu/67Cu for a theranostic approach to pretargeted radioimmunotherapy

Over the past decade, theranostic imaging has emerged as a powerful clinical tool in oncology for identifying patients likely to respond to targeted therapies and for monitoring the response of patients to treatment. Herein, we report a theranostic approach to pretargeted radioimmunotherapy (PRIT) based on a pair of radioisotopes of copper: positron-emitting copper-64 (⁶⁴Cu, t _1/2 = 12.7 h) and beta particle-emitting copper-67 (⁶⁷Cu, t _1/2 = 61.8 h). This strategy is predicated on the in vivo ligation between a trans-cyclooctene (TCO)-bearing antibody and a tetrazine (Tz)-based radioligand via the rapid and bioorthogonal inverse electron-demand Diels-Alder reaction. Longitudinal therapy studies were conducted in a murine model of human colorectal carcinoma using an immunoconjugate of the huA33 antibody modified with TCO (huA33-TCO) and a ⁶⁷Cu-labeled Tz radioligand ([⁶⁷Cu]Cu-MeCOSar-Tz). The injection of huA33-TCO followed 72 h later by the administration of 18.5, 37.0, or 55.5 MBq of [⁶⁷Cu]Cu-MeCOSar-Tz produced a dose-dependent therapeutic response, with the median survival time increasing from 68 d for the lowest dose to >200 d for the highest. Furthermore, we observed that mice that received the highest dose of [⁶⁷Cu]Cu-MeCOSar-Tz in a fractionated manner exhibited improved hematological values without sacrificing therapeutic efficacy. Dual radionuclide experiments in which a single administration of huA33-TCO was followed by separate injections of [⁶⁴Cu]Cu-MeCOSar-Tz and [⁶⁷Cu]Cu-MeCOSar-Tz revealed that the positron emission tomography images produced by the former accurately predicted the efficacy of the latter. In these experiments, a correlation was observed between the tumoral uptake of [⁶⁴Cu]Cu-MeCOSar-Tz and the subsequent therapeutic response to [⁶⁷Cu]Cu-MeCOSar-Tz.